The invention relates to a method of applying a boron layer to a steel substrate by means of a CVD process.
The invention also relates to a tool manufactured from steel and provided with a boron layer.
CVD (chemical vapor deposition) is a suitable method of coating surfaces. A known application is the provision of protective coatings on metal surfaces to render said surfaces resistant to, for example, wear, erosion, corrosion and/or oxidation. In a CVD process a solid substance is deposited on a (usually) heated substrate surface as a result of one or more chemical reactions in the gas phase. These reactions may occur on or near the substrate surface. The solid substance formed deposits on the surface of the substrate as a coating. CVD is a versatile method of applying coatings of metals and alloys, the composition, structure, grain size and purity of the coatings being adjustable. CVD is particularly suitable for applying coatings having a uniform layer thickness on objects having a complex shape. Known examples of coatings are nitrides, oxides, carbides and borides such as TiN, Al.sub.2 O.sub.3, SiC and TiB.sub.2. Elements such as boron (B) also can be provided on a substrate by means of CVD.
A method of the type mentioned in the opening paragraph is known from C. F. Powell et al., Vapor Deposition (John Wiley, New York, 1966) pages 349-350.
Boron has a number of attractive properties such as a high degree of hardness, a high melting point, a high resistance to wear and a satisfactory chemical resistance. For this reason, boron is a suitable choice for a wear-resistant coating on various steel tools. Since boron can be deposited from, for example, gaseous diborane, the reaction temperature during the CVD process can be kept below 500.degree. C. In this manner, degradation of the mechanical properties of the steel substrate can be prevented.
The provision of borides, carbides and nitrides on a substrate by means of a CVD process requires a process temperature of 1000.degree. C. or higher, which will adversely affect the mechanical properties of the steel substrate.
A disadvantage of the known method is the poor adhesion of the deposited boron layer on the steel substrate as a result of the relatively low reaction temperature. A steel tool coated with such a poorly adhering boron layer is unfit for many applications.